1. Field of the Invention
The present invention relates to a magnetic memory technology. More particularly, the present invention relates to an access operation to a magnetic memory cell for accurately reading and changing the data stored in the magnetic memory cell with low operation current.
2. Description of Related Art
Magnetic memory, such as magnetic random access memory (MRAM), is also a kind of non-volatile memory which has such advantages as non-volatility, high density, high read/write rate, and anti-radiation. Data of 0 or 1 is recorded in a magnetic memory through the quantity of the magnetoresistance produced by arranging the magnetizations of the magnetic materials, adjacent to a tunnel barrier insulating layer, in parallel or anti-parallel. When writing data, a magnetic memory cell is selected by crossing the magnetic fields induced by two current lines, such as bit lines (BL) and write word lines (WWL), and the magnetoresistance of the magnetic cell is changed by changing the direction of the magnetization of the free layer. When reading data, a current is passed through the selected magnetic memory cell and the digital memory data can be determined from the resistance of the magnetic memory cell.
FIG. 1 illustrates the basic structure of a magnetic memory cell. Referring to FIG. 1, to access a magnetic memory cell, crossing current lines 100 and 102 with certain currents passing through are used, and the current lines 100 and 102 are referred as, for example, word lines or bit lines, based on the operation manner thereof. The two current lines produce magnetic fields in two directions when currents are passed through the two current lines so that a magnetic field of required magnitude and direction can be obtained and supplied on the magnetic memory cell 104. The magnetic memory cell 104 is of stacked layer structure and includes a magnetic pinned layer having a fixed magnetization or a total magnetic moment in a predetermined direction. The data is read through the magnitude of the magnetoresistance. In addition, the data stored in the memory cell can be read through the output electrodes 106 and 108. The operation details of magnetic memory should be understood by those ordinarily skilled in the art therefore will not be described herein.
FIG. 2 illustrates the storage mechanism of a magnetic memory. In FIG. 2, the magnetic pinned layer 104a has a fixed magnetic moment direction 107. The magnetic free layer 104c is disposed above the magnetic pinned layer 104a, and the two layers are isolated by an insulating layer 104b. The magnetic free layer 104c has a magnetic moment direction 108a or 108b. If the magnetic moment direction 108a is parallel to the magnetic moment direction 107, the magnetoresistance produced denotes, for example, a binary data “0”; otherwise if the magnetic moment direction 108b is anti-parallel to the magnetic moment direction 107, the magnetoresistance produced denotes, for example, the binary data “1”.
FIG. 3 illustrates the relationship between the magnetoresistance (R) and the applied magnetic field (H) of a magnetic memory cell. The real line represents the magnetoresistance line of a single magnetic memory cell. However, a magnetic memory device may include a plurality of memory cells, and the magnitude of the reverse field of each memory cell may be different, thus, the magnetoresistance curve may be like the dotted line, which may result in access error. FIG. 4 illustrates the array structure of a conventional memory cell. The left diagram in FIG. 4 illustrates an array structure, for example, data is written into the memory cell 140 by supplying magnetic fields Hx and Hy in two directions. The right diagram illustrates the asteroid curve of the free layer. Within the real line area, the magnetization direction of the memory cell 140 will not be switched because the magnetic field is small. The magnetic field in a limited area outside the real line area is a suitable reversing magnetic field for reversing operation. The adjacent memory cell will be interfered if the magnetic field is too large, so large magnetic field is not suitable to be used. Thus, generally the magnetic field in the operation area 144 is serving as the operating magnetic field. However, since the other memory cells 142 will also sense the magnetic field supplied, the supplied magnetic field may also change the data stored in other memory cells 142 due to the change of the operating conditions of the adjacent memory cells 142. Accordingly, access error may be caused to the single free layer 104c in FIG. 2.
To resolve the foregoing problems, a magnetic free stacked layer 166 having FM/M/FM three layers structure is used to replace the single layer of ferromagnetic material in U.S. Pat. No. 6,545,906 to reduce interference to the adjacent memory cells when writing data, as shown in FIG. 5, and the two ferromagnetic metal layers 150 and 154 above and below the non-magnetic metal layer 152 are anti-parallel to each other to form a close flux. The magnetic pinned stacked layer 168 at the bottom is isolated from the magnetic free stacked layer 166 by a tunnel barrier layer (T) 156. The magnetic pinned stacked layer 168 includes a top pinned layer (TP) 158, a non-magnetic metal layer 160, and a bottom pinned layer (BP) 162. The top pinned layer and the bottom pinned layer have fixed magnetizations. In addition, a base layer 164, for example, an anti-ferromagnetic layer, is disposed at the bottom.
As to the magnetic free stacked layer 166 of three-layer structure, the magnetic anisotropic axes of the first and the second write lines corresponding to the free stacked layer 166 form an including angle of 45°, and the direction of the magnetic anisotropic axis thereof is the direction of easy axis. Accordingly, the first and the second write lines can respectively supply a magnetic field having an angle of 45° from the easy axis to the free stacked layer 166 successively, so as to rotate the magnetization of the free stacked layer 166. FIG. 6 illustrates the timing of supplying magnetic fields. In FIG. 6, the top diagram illustrates the relative direction of the easy axis (denoted by the double-arrow) to the magnetic field. The bottom diagram illustrates the timing of supplying currents to the first and the second write lines. Wherein, current I1 represents that a magnetic field in 45° to the easy axis will be produced, namely, the perpendicular axis in the top diagram, and current I2 represents that a magnetic field in −45° to the easy axis will be produced, namely, the horizontal axis in the top diagram. The magnetization directions of the two ferromagnetic layers 150 and 154 of the free stacked layer 166 are reversed in direction based on the timing of supplying the currents. This timing of current supplying is accomplished with two stages, thus, the operation is also referred to as toggle mode operation. The magnetization directions of the two ferromagnetic layers 150 and 154 of the free stacked layer 166 are reversed once after each toggle mode operation. Since the magnetization direction of the top pinned layer 158 is fixed, the magnetization direction of the bottom ferromagnetic layer 154 may be parallel or anti-parallel to the magnetization direction of the top pinned layer 158, so that a binary data can be stored.
FIG. 7 illustrates the correspondence of the magnetizations on the two ferromagnetic layers 150 and 154 of the free stacked layer 166 to the magnitude of the external magnetic field. Referring to FIG. 7, in situation (a), the thin arrows represent the magnetization directions of the two ferromagnetic layers 150 and 154 of the free stacked layer 166. In situation (b), the two magnetization directions are not changed when the external magnetic field H (bold arrow) is small. The two magnetization directions reach a balance with the magnetic field H when the external magnetic field H is increased to a certain value, so that an opening angle is formed, here the range of the magnetic field is the toggle operation area in toggle mode, and the rotation of the magnetization thereof is by changing two magnetic fields perpendicular to each other based on a particular timing (referring to FIG. 6). Accordingly, the magnetization is rotated stage by stage. However, the two magnetization directions are always led to the same direction of the magnetic field H if the magnetic field H is too large, which is not an appropriate operation area.
FIG. 8 illustrates the reversing mechanism of supplying the magnetic field produced by the operation current in FIG. 6 to a memory cell. Referring to FIG. 8, during time section t0, no magnetic field is supplied, thus, the two magnetizations on the two free layers are anti-parallel to each other. During time section t1, a magnetic field H1 in 45° to the easy axis is supplied to the free stacked layer. Here the two magnetizations are rotated based on the direction of the magnetic field supplied. During time section t2, a magnetic field H2 is supplied at the same time. The direction of the magnetic field H2 forms an angel of −45° to the easy axis. Thus, if the magnitudes of the two magnetic fields are the same, the direction of the total magnetic field is on the easy axis. Here the two magnetizations are rotated again. Next, during time section t3, it stops supplying the magnetic field H1. Here the total magnetic field is supplied by the magnetic field H2, so that the two magnetizations are rotated again. It should be noted that during time section t3 the two magnetizations have been approximately reversed corresponding to the easy axis. Accordingly, during time section t4, the two magnetizations return to the direction of the easy axis in anti-parallel status when the external magnetic field disappears, so that the two magnetizations are reversed.
FIG. 9 illustrates the operating areas corresponding to external magnetic fields. Referring to FIG. 9, the operation area on the magnetic field coordinate corresponding to the toggle operation mode in FIG. 8 is toggle area 97. There are also non-switching area 92 and direct area 95. The direct area 95 is disposed between the non-switching area 92 and the toggle area 97 and the details thereof are not described herein.
The U.S. Pat. No. 6,633,498 provides a design for reducing operation magnetic field. FIG. 10 illustrates the design of reducing operation magnetic field. Referring to FIG. 10, the conventional design is to adjust the magnitudes of the total moments 170 and 172 of the top pinned layer 158 and the bottom pinned layer 162 of the magnetic pinned stacked layer so as to produce a leakage magnetic field. The leakage magnetic field will produce a biased magnetic field HBIAS to the free stacked layer, as shown in the figure. The starting point of the toggle operation area is closer to the magnetic field zero. Wherein, the magnitudes of the magnetic moments can be adjusted by simply adjusting the thicknesses of the pinned layers.
In the conventional technology described above, the magnetic field HBIAS can not be increased unlimitedly. According to the research of the present invention, in the conventional technology, if the biased magnetic field HBIAS is too strong, at least the data stored in the memory cell is directly interfered, which may cause data access failure.